I think is the best solution. Udev itself was overwriting your symlink. The dirty solution would be add ln -s /dev/ttyACM0 /dev/modem to the end of your /etc/rc.d/rc.local, but you are more elegant than that

Logged

"There is a concept which corrupts and upsets all others. I refer not to Evil, whose limited realm is that of ethics; I refer to the infinite."Jorge Luis Borges, Avatars of the Tortoise. --Jumalauta!!